Implementing a Circular Buffer (Sliding window) in MySQL

Question

I intend to store some Java objects in a MySQL database, accompanied by a timestamp. These objects should be kept in a Sliding Window fashion (also known as Circular Buffer), meaning that only the last N items from a specific type should be kept.

My general idea is a an INSERT trigger:

DELETE FROM pixels WHERE type="type_of_new_pixel"
                         AND id NOT IN 
                         (SELECT id FROM pixels 
                          WHERE type="type_of_new_pixel"
                          ORDER BY timestamp DESC LIMIT N);

• Is triggger the right way to go? How do I add the trigger to the table?
• In order to increase performance, I would like to activate the trigger every M INSERTs. I will have some extra items in my list, but that's OK. How do I implement this? Will a id % 100 == 0 check do?